1. Field of the Invention
The present invention relates to a surface light emitting apparatus emitting laser light, and more particularly, to a method for illuminating uniformly a backlight panel of a laser display by means of incoherent laser beam shaping and a refractive optics.
2. Description of the Related Art
In recent years the need for flat panel displays has increased, in applications such as laptop screens, cellular phones screen and other portable devices, desk-top monitors, TV, etc. With the emergence of light valve type displays (e.g. liquid crystal displays (LCDs)) as one of the leader technologies in the flat panel industry, efforts are being made on improving backlighting: emitters as well as lighting techniques. The requirements on emitting materials include high brightness, efficiency, contrast and accurate RGB chromaticity, while device technology requires factors such as thin form, scalability (large to micro-displays), and low cost.
Until now, backlighting relied on broadband emitters such as fluorescent lamps as they are very bright and efficient white light emitters. However, their broad spectrum requires that most of the light generated be filtered out because it is at the wrong wavelength. About 70% of the light is thusly thrown away and the filtering technology involved to obtain the correct RGB TV colors is complex and expensive. In addition to decreasing the overall efficiency of the process, the unused emission results in undesirable heating of the display, triggering a search for alternatives.
With the recent development of efficient, high power blue, green and red laser sources, laser-based backlighting is emerging as a possible solution. These lasers provide narrow spectrum at the RGB TV colors, eliminating the need of complex filtering technology, and can be packaged into small size units, making them suitable for flat panel displays. In addition to being more energy efficient than lamp-based backlights, another obvious advantage of laser backlights is much broader color gamut coverage that gives much more vivid color displays. They also provide a more environmentally friendly alternative to mercury-based lamps.
However, laser light also differs with lamps in that it has different radiometric properties: laser light emits in a determined direction and generally has a Gaussian intensity profile, whereas lamps are Lambertian emitters (i.e. emit uniformly in all directions). Therefore new light shaping techniques adapted to laser light need to be developed in order to use efficiently laser sources in backlighting applications. One requirement is to illuminate uniformly the back panel (light guide plate) of LCD displays with wasting as little light as possible.
One technique is to use a light-guide (e.g., optical fiber) leaking light all along a side of the back panel, taking advantage of the possibility of efficiently coupling laser light into a fiber. However the “leaks” need to be carefully controlled to obtain uniform illumination while not wasting unused light (all light needs to have been leaked within the back panel at an incidence surface that is an end face of the back panel). This can lead to a high degree of complexity in design and fabrication. It is desirable to achieve a simpler design where uniformity can be more easily controlled while keeping high efficiency and that for any sizes (small and large displays).
Another technique is to use light integrators type of filters to homogenize the light (spatial light modulators for instance), but these are mainly used with broad-band types of white light, and not recommended for laser light. For instance, a light integrator can improve beam uniformity, but is not intended to scale the illumination to larger displays, and more optics has to be introduced, which lowers the efficiency of the method.